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Re:History Sniffing: How YouPorn Checks What Other Porn Site... by JacobAldridge on Thursday December 02, 2010 @10:26AM (#1962376)
Here's Mozilla's linked post on this, from March of this year - http://blog.mozilla.com/security/2010/03/31/plugging-the-css...

To support your statement, I haven't found anything saying this has been removed.

Re:History Sniffing: How YouPorn Checks What Other Porn Site... by fhars on Thursday December 02, 2010 @10:53AM (#1962448)
There seems to be a partial fix in the 4.0 codebase: https://bugzilla.mozilla.org/show_bug.cgi?id=147777#c259
Re:History Sniffing: How YouPorn Checks What Other Porn Site... by sid0 on Thursday December 02, 2010 @12:34PM (#1962845)
Yeah, sites like http://www.didyouwatchporn.com/ and http://ha.ckers.org/weird/CSS-history-hack.html fail to work in Firefox 4. The followup bugs are more to do with tuning what is allowed and what isn't, e.g. https://bugzilla.mozilla.org/show_bug.cgi?id=559722.